I got this DLC yesterday morning and I was completely disappointed by it.
The new campaign is short, very short. The overly hyped meeting between the characters from the two games was nothing more than a 'hi, bye' type of thing, very little interaction between the two groups. The campaign itself would have been better if it were longer, 3 maps one being another stupid fill the generator. The did manage to make it feel more like a L4D one map thought with 2 maze like maps instead of the more straight run from point A to B like in L4D 2. Mutation, a new mode, has promise. Each week a new theme will be introdeuced, this weeks' is Realism VS., but really, how long will Valve keep this going with new modes? There's a single player option aside from online now, I have no idea why that was seperated.
All in all, it was a huge let down. Even the new achievements can be won during the first 5 minutes of the game except for the Charger one, which has to be done in VS..